1. Field of the Invention
The present invention relates to a DC/DC conversion apparatus that includes a LLC full-bridge circuit.
2. Description of the Related Art
In the prior art, a switch power supply is a power supply that utilizes a modern power electronic technology to control a ratio of a turn-on time and a turn-off time of a switch and maintain a stable output voltage, in which a DC/DC conversion apparatus, i.e., direct current-direct current conversion circuit, is a voltage transformer that effectively converts a DC input voltage into a fixed DC output voltage. Generally, the DC/DC conversion apparatus is divided into three types: a boost DC/DC transformer, a buck DC/DC transformer, and a boost-buck DC/DC transformer, and three types of control may be utilized according to requirements. Specifically, by utilizing energy storage characteristics of a capacitor and an inductor, high-frequency switching actions are performed by a controllable switch (MOSFET, etc.), inputted electric energy is stored in the capacitor or the inductor, and the electric energy is released to a load so as to provide energy when the switch is turned off. Its ability to output power or a voltage is related to a duty cycle, i.e., a ratio of a turn-on time of the switch and the entire cycle of the switch.
However, as the power electronic technology is developing rapidly, requirements, such as more high-frequency operation, high conversion efficiency, high power density, low noise and other requirements, have been proposed to a switch power supply.
FIG. 8 shows an existing DC/DC conversion apparatus 100 that includes a LLC full-bridge circuit. As shown in FIG. 8, the DC/DC conversion apparatus 100 includes a direct-current (DC) voltage source V10, four switch elements Q1˜Q4, an oscillation circuit 20 including an inductor Lr and a capacitor Cr, and a transformation circuit 40 including a transformer 30 and a rectification circuit. In the DC/DC conversion apparatus 100, turn-on and turn-off of the individual switch elements Q1˜Q4 are controlled, so as to control energy to be transmitted from a primary side Tr1 to secondary side Tr2 of the transformer 30.
For the individual switch elements Q1˜Q4 in the DC/DC conversion apparatus 100 as shown in FIG. 8, their control sequences are shown in FIG. 9.
As shown in FIG. 9, a duty cycle of each switch element Q1˜Q4 is 50%. At time t0, the switch elements Q1 and Q4 are turned on, the switch elements Q2 and Q3 are turned off, and a voltage Vc +− applied to the oscillation circuit 20 including the inductor Lr and the capacitor Cr is a positive value. At this moment, a current ILLC flowing through the oscillation circuit 20 is a positive value and increases gradually. Then, at time t1, the switch elements Q2 and Q3 are turned on and the switch elements Q1 and Q4 are turned off. At this moment, the voltage Vc+− applied to the oscillation circuit 20 instantly becomes a negative value because the voltage is varying intermittently. However, since the current is varying consecutively, as shown in FIG. 9, at time t1, when the voltage Vc+− applied to the oscillation circuit 20 instantly becomes a negative value, the current ILLC flowing through the oscillation circuit 20 is still a positive value although it decreases gradually. In other words, from time t1 (i.e., time of switching the switches) until a time of the current ILLC flowing through the oscillation circuit 20 being reduced to zero, the current ILLC flowing through the oscillation circuit 20 has a different phase from the voltage Vc+− applied to two terminals of the oscillation circuit 20. The result is that, since energy to be outputted to the secondary side Tr2 of the transformer 30 is a product of the voltage Vc+− and the current ILLC, as shown in FIG. 9, the energy to be outputted to the secondary side of the transformer 30 is negative (i.e., the energy flows reversely from the oscillation circuit 20 to the DC voltage source V10) within a time period of A→B, and the energy will oscillate between the DC voltage source V10 and the oscillation circuit 20 after the time period of A→B. The oscillation between the DC voltage source V10 and the oscillation circuit 20 and a resistance present on a current path of the oscillation circuit 20 will result in an unnecessary loss.
Likewise, at time t2, the switch elements Q1 and Q4 are turned on and the switch elements Q2 and Q3 are turned off. At this moment, since the voltage is varying intermittently, the voltage Vc+− applied to the oscillation circuit 20 instantly becomes a positive value. However, since the current is varying consecutively, as shown in FIG. 9, at time t2, when the voltage Vc +− applied to the oscillation circuit 20 instantly becomes a positive value, the current ILLC flowing through the oscillation circuit 20 is still a negative value although it increases gradually. The result is that, as shown in FIG. 9, the energy to be outputted to the secondary side Tr2 of the transformer 30 is negative (i.e., the energy flows reversely from the oscillation circuit 20 to the DC voltage source V10) and oscillates between the DC voltage source V10 and the oscillation circuit 20 within a time period of C→D. A resistance present on the current path of the oscillation circuit 20 will result in an unnecessary loss.
In addition, a gain perspective should also be considered. Assume that a gain of the DC/DC conversion apparatus 100 is 1, switching frequencies of the individual switch elements Q1˜Q4 in the DC/DC conversion apparatus 100 are equal to a resonance frequency of the oscillation circuit 20. At this moment, in an ideal state, a loss will not be generated in the DC/DC conversion apparatus 100. However, if the gain is less than 1, an input voltage Vin is certainly greater than an output voltage Vout. Since the duty cycles of the switch elements Q1˜Q4 are 50%, ILLC=Iout (i.e., the current ILLC flowing through the oscillation circuit 20 is equal to an output current) and input energy (i.e., a product of Vin and ILLC) is certainly greater than output energy (i.e., a product of Vout and Iout). Wherein this extra portion (i.e., a value of Vin*ILLC−Vout*Iout) has been consumed in the DC/DC conversion apparatus 100.
In other words, in the existing DC/DC conversion apparatus 100 as shown in FIG. 8, turn-on and turn-off of the individual switch elements Q1˜Q4 are controlled by the duty cycle 50%, such that a portion of the energy flows reversely to the DC voltage source V10 and, thereafter, flows back and forth between the oscillation circuit 20 and the DC voltage source V10, which results in a loss and a reduced output power, such that the gain of the DC/DC conversion apparatus 100 also decreases, as shown in FIG. 9.
On the other hand, in the DC/DC conversion apparatus that includes a LLC full-bridge circuit, there is also a problem of a loss of a switch (i.e., a MOSFET). For the problem of switching loss, a soft-switching technology is usually utilized in the present technical field.
Soft-switching is in contrast to hard-switching. Generally, resonance is introduced before and after the process of the turn-on and the turn-off, such that a voltage before the switch is turned on is firstly reduced to zero and a current before the switch is turned off is firstly reduced to zero, which can eliminate an overlap of the voltage, the current during the turn-on and the turn-off and reduce their variance ratio so as to greatly reduce or even eliminate the switching loss. At the same time, a variation ratio of a voltage and a current of the switch during the turn-on and the turn-off is restricted by the resonance process, which significantly decreases the noise of the switch.
For the process of turn-off of the switch, an ideal soft turn-off process is such that the current is firstly reduced to zero and then the voltage increases slowly to an off-state value. At this moment, a turn-off loss of the switch is approximately zero. Since the current of the device before the turn-off has been reduced to zero, the problem of inductive turn-off has been solved. This is usually referred to as a zero current switch (ZCS). In addition, for the process of turn-on of the switch, an ideal soft turn-on process is such that the voltage is firstly reduced to zero and then the current increases slowly to an on-state value. At this moment, turn-on loss of the switch is approximately zero. Since the voltage of a junction capacitance of the device is also zero, the problem of capacitive turn-on has been solved. This is usually referred to as a zero voltage switch (ZVS).
In the prior art, in order to reduce the loss of the switch when it is turned on or even achieved the zero current switch (ZCS) and/or the zero voltage switch (ZVS), turn-on and turn-off sequences of the individual switch elements Q1˜Q4 have to be adjusted appropriately.